uspace/app/blkdump: fix argv null deref

[mcimerman]

Previous if conditions might increment argv, and the next conditions can dereference the dereference *(*argv) (instr_cmp) without checking.

Reproduce crash: for example: blkdump --offset 16

[le-jzr]

Thanks!
At first I thought it's a regression due to me changing str_cmp(), but turns out the previous implementation didn't handle null either.